Genetics Northern California

Duchenne Muscular Dystrophy

General Information
Duchenne muscular dystrophy (DMD) is a genetic disorder that affects the muscles. The condition is caused by a mutation in the dystrophin gene. A person with DMD is completely (or almost completely) missing a protein, called dystrophin, which is normally made inside the body. Dystrophin is needed to keep muscles healthy. Without this protein, the muscles slowly break down. DMD mainly affects the skeletal (voluntary) muscles and the cardiac (heart) muscles. Mutations in the dystrophin gene can also cause a related condition called Becker muscular dystrophy. In a person with Becker muscular dystrophy, the dystophin protein is made, but the amount is less than normal. Symptoms are similar to DMD, except they usually start later in life.
 
Duchenne muscular dystrophy usually affects males because it is an X-linked condition. About 1 in every 3500 to 5000 boys will have DMD and all racial and ethnic groups are affected equally. Females can be carriers of DMD and may show no signs or have only minor muscle weakness. Very few women have serious medical complications related to DMD. More than half of the time, DMD is inherited from the mother, who may be unaware she is a carrier for the condition. About 1 in every 3 cases of DMD happens due to a new genetic change (de novo mutation) during conception. When a woman is a carrier of DMD, each male child has a 50% chance of being affected with DMD and a 50% chance of not being affected. Each female child from a woman with a DMD mutation has a 50% chance of being a carrier of DMD and a 50% chance of not being a carrier. 

Features of Duchenne Muscular Dystrophy
The main features of DMD are muscle weakness and muscle wasting (muscle loss). DMD is a progressive condition, with muscle weakness becoming more severe as a boy grows older. Boys with DMD generally start walking later than average and also have more trouble with activities like running, jumping, climbing stairs, or throwing a ball. Gradually, the muscles in the legs weaken to the point that a wheelchair is needed. For boys with DMD, wheelchair-dependence often happens by the age of 12 years, but the exact age is not always predictable. Over time, the heart muscles and the breathing muscles also become weaker. The most common cause of death is related to difficulty breathing (pulmonary involvement) or the breakdown of the heart muscle. The average life expectancy for a boy with DMD is the late 20’s; however a few young men with DMD live into their 30’s.

Mental retardation is not usually a feature of DMD, however some boys with this condition require special education services through their school years. The need for special education services is often due to difficulties in developing social and cognitive skills. 

Diagnosing Duchenne Muscular Dystrophy
Duchenne muscular dystrophy is suspected based on clinical features. The features a doctor looks for include how a boy walks or runs, how he goes up and down stairs, how he stands up when sitting on the ground, and the way his leg muscles appear. The diagnosis is confirmed by testing creatine kinase (CK) levels. CK is a protein found in the blood that shows how fast the muscles are breaking down. In boys with DMD, the amount of CK is generally 10 times higher than normal. A sample of the muscles (muscle biopsy) was once used to help confirm the diagnosis. Since genetic testing became available, a muscle biopsy is rarely necessary. A family history of DMD can be helpful in making an early diagnosis.

Genetic Testing
Genetic testing may be done to confirm a diagnosis or to identify the specific mutation in a family. Nearly all males with DMD have a mutation in the dystrophin gene that can be detected. If the specific mutation of someone with DMD is known, carrier testing can be offered to females in the family and prenatal diagnostic testing is available for future pregnancies.

Treatment of Duchenne Muscular Dystrophy
There is currently no cure for DMD, but medical treatment may help slow the progression of the disease. Steroid therapy, when timed correctly, can optimize muscle strength and potentially prolong the ability to walk. In addition, all boys with DMD should be monitored by a cardiologist. The cardiologist may use medications to manage cardiac symptoms. Female carriers of DMD should also have a cardiac evaluation. Young boys with DMD need close pediatric follow-up to watch for delays in development. A special education evaluation should be requested when there are concerns about development.

Researchers are currently looking for more effective treatments. To learn more about research opportunities contact one of the support organizations below or go to Clinicaltrials.gov.


Support Resources
Muscular Dystrophy Association- DMD Page
http://www.mda.org/disease/dmd.html

Parent Project Muscular Dystrophy
http://www.parentprojectmd.org/

References
Gene Reviews on Dystrophinopathies:
http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=gene&part=dbmd

NIH: Genetics Home Reference on Duchenne and Becker Muscular Dystrophy:
http://ghr.nlm.nih.gov/condition/duchenne-and-becker-muscular-dystrophy

American Academy of Pediatrics: Cardiovascular Health Supervision for Individuals Affected by Duchenne or Becker Muscular Dystrophy
http://aappolicy.aappublications.org/cgi/reprint/pediatrics;116/6/1569.pdf


Written by:     Elizabeth Stark, MS
Reviewed by: Kim Barr, MS CGC
                     Emily Chen, MD, PhD
                     Joyce Kobori, MD
                     SJO Genetics

Last Edited:       September 24, 2010
Last Reviewed:  September 24, 2010